With some help from moparherb, I set up a throttle cut off switch, and got an added bonus.
Been looking for a way to get proportional flaps on the throttle stick, and now I have it.
Here's how:
First using moparherb's throttle cut off mix, tied to the mix switch (aileron rudder mix which I do not use)
I use mix 3
make it a THRO to THRO mix
POS 0 Should be 0% in both Positions
POS 1 make both settings -100% that is minus 100%
you do that by changing the position of throttle stick
Set Offset to -200 that is minus 125
make sure that "mix" is set to position 1

Now when you put the throttle hold switch in position 1 (toward you) the motor will be disabled.

An added bonus..I used another mix Throttle>Gear to give me proportional flaps on the throttle stick when the throttle cut off switch is off. That's something I've been wanting for a long time! So now I can launch, then hit the throttle cut off and the flaps are at 0. Pulling back the throttle deploys the flaps proportionately to full flaps when the stick is full back. Awesome!

Here are the mix settings for my RP, for proportional throttle:
I used mix 5:
THRO>GEAR OFF
RATE: 0%
+63%
SW:MIX
OFFSET:86